This invention relates to a method of recovering metals from low grade ores and residues. More particularly, the invention is concerned with increasing the economic viability of recovering base or other metals by means of cyanide leaching of low grade ores and residues by providing a method for the effective recovery of cyanide, contained in a lixiviant to the leach process, which allows for the recovered cyanide to be recycled.
Without being restrictive “low grade ores”, as used herein, includes oxide and sulphide ores, especially ores containing high acid-consuming gangue materials such as calcite, dolomite, calcium, magnesium, aluminium, manganese and iron.
The recovery of base metals such as copper, nickel, cobalt and zinc from low grade ores by means of sulphuric acid leaching is often considered uneconomical, at least for the following reasons:    1. the amount of acid, which is a costly reagent, consumed during the leaching of gangue materials such as the aforementioned is relatively high;    2. base metal ores or residues containing sulphides are not leached efficiently in sulphuric acid and, for most sulphide materials, an oxidant such as ferric must be employed. The cost of the oxidant is normally unacceptably high; and    3. during sulphuric acid leaching precious metals such as gold, silver and platinum are not leached and an acid residue must be further processed by cyanidation to recover these metals.
Although the cyanidation of low grade ores can be very effective in recovering base metals and precious metals, the high levels of free cyanide that are required to ensure effective leaching have a detrimental effect on the economic viability of the process.
Several methods for recovering cyanide, following a cyanidation process, have been developed so that the recovered cyanide can be recycled in order to make the cyanidation process more economical. One method, referred to as the SART process, used for recovering gold from copper-gold ores, has shown promise.
A basic flow sheet of the SART process is shown in FIG. 1. The process includes the following steps: leaching the ore with cyanide to form a solution which is subjected to a first solid/liquid separation step to produce a pregnant leach solution (PLS) and solids; acidification of the PLS using H2SO4; followed by sulphidisation by means of the addition of H2S, to produce a solution containing a metal sulphide complex, which is subjected to a second solid/liquid separation step, such as activated carbon filtration or ion exchange, to form a filtrate and a filtered liquor. Copper, gold and silver are recovered from the filtrate and the pH of filtered liquid is adjusted before recycling to the leaching step. The solids from the first solid/liquid separation step are washed and subjected to a cyanide destruction step to form a detoxified product which is disposed of in a slimes dam.
The SART process is primarily used for gold recovery. To the applicant's knowledge the SART process has not been considered for the treatment of low grade base metal ores or residues, for the quantity of base metal which would be recovered would not normally justify the high cost of the reagents.
An object of the present invention is to provide a method which, at least partially, addresses the problem associated with these high costs.